1. Field of the Invention
The present invention relates to a muting circuit.
2. Description of the Related Art
An electronic volume that electronically controls the volume of speakers is typically provided with a muting circuit that produces no-sound condition (mute) by temporarily cutting off an audio signal when the noise superimposed upon the audio signal is detected or when a mute button is pressed. The pitch and the volume of an audio signal are known to have characteristics to be adjusted by frequency control and by amplitude control, respectively. With the application of such the characteristics of an audio signal, a muting circuit executes a process to produce no-sound condition (hereinafter, referred to as a muting process) by fixing a DC level at a predetermined level to eliminate the amplitude of the audio signal (see FIG. 4). As the predetermined DC level, “AC-GND” in an electronic volume is generally employed. The AC-GND means a GND applied for an AC signal that indicates a constant DC level as the amplitude center of an AC signal waveform.
FIG. 5 is a schematic illustrating the configuration of an electronic volume 10 equipped with a conventional muting circuit 30. As shown in the figure, the electronic volume 10 is composed mainly of an attenuation amount control circuit 20 that controls the attenuation amount of an AC input signal IN (audio signal) for volume control, and a muting circuit 30, that is equipped with analogue switches SW1 and SW2 that are switched on and off complimentarily by the electronic control of a microcomputer (not shown), that executes the muting process for the AC input signal IN after attenuation amount control (see, e.g., Japanese Patent Application Laid-Open Publication No. 1999-327579).
In normal state where muting process is not executed, the muting circuit 30 outputs the attenuation amount-controlled AC input signal IN as is as an AC output signal OUT by switching the analogue switches SW1 on and SW2 off. Here, the amplitude center of the AC input signal IN is to indicate a constant DC level Va. On the other hand, when muting process is executed, the muting circuit 30 outputs an AC-GND level Vb by switching the analogue switches SW1 off and SW2 on (see sections a and b of FIG. 6). Consequently, no-sound condition can be produced since the DC level of the AC output signal OUT is fixed at the AC-GND level Vb.
In the case of a conventional muting circuit such as the muting circuit 30 shown in FIG. 5, an AC input signal IN without muting process contains a DC offset Vx, that is a difference voltage between the AC-GND level Vb, due to the internal circuit characteristics of the electronic volume 10 and/or due to the manufacturing variation of ICs that compose the attenuation amount control circuit 20. The DC voltage of an AC output signal OUT is fluctuated by the DC offset Vx when the analogue switches SW1 and SW2 are switched-over between on and off, and noise is generated by such DC voltage fluctuations.
In the ideal state of an AC output signal OUT as shown in section c of FIG. 6, where a DC voltage fluctuation is induced momentarily at the start or the end of muting, the noise generated by the DC voltage fluctuation becomes a sound out of the audible frequency range, and therefore, the sound is harmless in being inaudible to humans, and thus it is not problematic. However, as an AC output signal OUT fluctuates slowly at the start or the end of muting as shown in section d of FIG. 6 due to the internal circuit characteristics of the electronic volume 10 and/or due to the manufacturing variation of ICs, this has been problematic, generating abrasive noise within the audible frequency range.